Field of the Invention
The invention concerns a method for actuating shimming in a magnetic resonance apparatus, as well as a control computer and magnetic resonance apparatus designed to implement such a method.
Description of the Prior Art
In a magnetic resonance (MR) apparatus, also called a magnetic resonance tomography system, a subject to be examined, in particular the body of a patient, is exposed to a relatively strong basic magnetic field with the use of a basic field magnet, for example 1.5 or 3 or 7 tesla. In addition, gradient pulses are activated by a gradient coil arrangement. A radio-frequency antenna also emits high-frequency radio-frequency pulses, for example excitation pulses, from suitable radiators that cause the nuclear spins of specific atoms excited to resonance by these radio-frequency pulses to be tilted by a defined flip angle relative to the magnetic field lines of the basic magnetic field. Upon relaxation of the nuclear spins, radio-frequency signals, known as magnetic resonance signals, are emitted, and are received by suitable radio-frequency antennas and then processed further. Finally, the desired image data can be reconstructed from the raw data acquired in this manner.
Therefore, for a specific scan, a specific magnetic resonance sequence, also referred to as a pulse sequence, is used, which is composed of a series of radio-frequency pulses, for example excitation pulses and refocusing pulses and gradient pulses which are to be emitted and that are suitably coordinated with specific gradient pulse shapes in different gradient axes along different spatial directions. Chronologically coordinated therewith, readout windows are set that predetermine the periods of time in which the induced magnetic resonance signals are acquired (detected).
During magnetic resonance imaging with a magnetic resonance scanner, the homogeneity of the basic magnetic field in the examination volume of the scanner is of great importance. Even small deviations in homogeneity can result in large deviations in the frequency distribution of the nuclear spins, so that lower-quality magnetic resonance image data are acquired. To improve the homogeneity in the examination volume, a magnetic resonance scanner typically has an adjustable shim coil arrangement. Such a shim coil arrangement has electrical shim coils that are fed from an amplifier with different shim currents so as to generate different compensation magnetic fields in order to improve the homogeneity.
The emission of specific gradient pulse shapes, defined by specifications of the magnetic resonance sequence, by the gradient coils of the scanner can result in unwanted eddy current fields in a reception volume of the magnetic resonance sequence. In this context, the operation of the gradient coils can cause eddy currents in all electrically conductive components of the magnetic resonance scanner. Eddy currents are particularly likely to occur in the shielding of the basic field magnet of the scanner that is made, for example, of aluminum. The eddy current fields that occur in the reception volume can result in a loss of quality of the magnetic resonance images acquired by the magnetic resonance sequence. For example, with certain magnetic resonance sequences, fat suppression can be impaired by the eddy current fields.
Although a typical gradient coil arrangement of a magnetic resonance apparatus is designed to ensure the eddy currents caused by the operation of the gradient coils thereof, and hence the eddy current fields in the reception volume of the magnetic resonance sequence, are minimized, production tolerances during the manufacture of the gradient coils can result in a deviation from the desired shape of the actual conductor path of the gradient coil in the conductor guide. For example, primary conductor patterns can be shifted in an axial direction relative to secondary conductor patterns. A shift of this kind can result in a loss of symmetry of the conductor guide in the gradient coil arrangement and hence produce integral terms in the gradient field and eddy current field of the gradient coil arrangement. Thin gradient coil arrangements can be more sensitive to a shift of this kind. While the disruptive terms in the gradient field can typically be ignored, it not usually possible to ignore the integral terms in the eddy current field of the gradient coil arrangement. Typical integral terms in the eddy current field of the gradient coil arrangement are, for example, directed along the z-axis (A (2,0) terms), along the x-axis (A(2,1) terms) or along the y-axis (B(2,1) terms).